Table 2 Evidence for introgression or shared ancestral polymorphism

Species are know to hybridize under greenhouse conditions and in the wild

Species cannot hybridize under greenhouse conditions, and/or little evidence of hybridization in the wild

Strength of reproductive barrier dictates potential for hybridization and introgression

Tightly linked genes show similar patterns of gene flow between species

Lack of correlation between allelic states at linked genes

Recombination less likely between linked genes, and introgressed linkage blocks are likely to be maintained

Different patterns of genetic distances between species at different loci, with the greatest degree of divergence between loci contributing to species-level differences

Equal divergence across different loci

Selection pressures different for different loci, with strong diversifying selection at loci underlying species-specific traits

Populations in sympatry or parapatry show less genetic divergence than those that occur in allopatry

Equivalent levels of divergence across species range

Where introgression occurs a cline of shared genes may be expected between species

Phylogenetic incongruency between uniparentally and biparentally inherited markers

Congruency between the phylogeny derived from uniparentally inherited markers and the maximum clade credibility tree from biparentally inherited markers

Uniparentally inherited markers (thus with a low effective population size) are particularly susceptible to introgression